1. Field of the Invention
This invention relates to the field of integrated circuits. More particularly, this invention relates to power-on control of integrated circuits and a programmable comparator.
2. Description of the Prior Art
It is known to provide integrated circuits with one or more virtual power rails and one or more virtual ground rails. These virtual rails are selectively connected or disconnected to the main power rails and the main ground rails respectively by header transistors and footer transistors. This technique is useful in reducing power consumption, particularly due to leakage, when a block/domain within an integrated circuit is not required to be active and accordingly can be powered down and isolated from the power supply and the ground by use of the header and footer transistors.
FIG. 1 of the accompanying drawings schematically illustrates such an integrated circuit 2 including a logic block 4 drawing power from a virtual supply rail 6. The virtual supply rail 6 is connected to a main supply rail 8 via both a strong header transistor 10 and a weak header transistor 12. The strong header transistor 10 has a high conductance and the weak header transistor 12 has a comparatively low conductance. When it is desired to power-on the supply to the logic block 4 this operation is conducted in two phases. First the weak header transistor 12 is switched on to gradually raise the voltage level of the virtual supply rail 6 toward that of the main supply rail 8. The weak header transistor 12, as a consequence of its relatively low conductance, generates comparatively small inrush currents into the virtual supply rail 6 and the logic block 4 thereby helping to avoid damage and erroneous operation which could otherwise occur if the strong header transistor 10 were switched on from the outset resulting in an excessively large current and/or an undesirable dip in the main supply rail voltage, which could cause errors in other portions of the integrated circuit. When the voltage on the virtual supply rail 6 has reached a predetermined trigger level, a power-on controller 14 can at that point switch on the strong header transistor 10, which will be capable of meeting the power demands of the logic block 4 when it commences processing.
It will be appreciated by those in this technical field that the header transistors 10, 12 that are illustrated are only examples of a large number of such header transistors present on the power grid of the integrated circuit as a whole with many of these transistors being provided in parallel to link the main supply rail (S) 8 to the virtual supply rail (S) 6. The example of FIG. 1 has shown a weak header transistor 12 to be used first to bring the virtual supply rail 6 nearly up to its operating level with the strong header transistor 10 then being switched on. It may be the case that all of the header transistors are of the same strength (conductance) but the same effect is created by switching on a small proportion of the header transistors provided in parallel between the main supply rail 8 and the virtual supply rail 6 so as to limit the inrush current and the voltage drop on the main supply rail 8. When the virtual supply rail 6 has almost reached its operating voltage level, then the remainder of the header transistors can be turned on.
FIG. 2 is of the accompanying drawings a graph showing the variation in the voltage on the virtual supply rail 6 with time during the power-on process. In the first portion of the power-on process the voltage rises relatively slowly through the action of the weak header transistor 12 until a trigger voltage Vtrig is reached. When this trigger voltage Vtrig is reached, the strong header transistors 10 are switched on and the voltage on the virtual supply rail 6 rapidly driven to the full operational level Vdd.
When the strong header transistors 10 are turned on as the trigger voltage is reached the relatively large currents which can flow at that time (even given the precharging by the weak header transistor 12) can result in a temporary reduction in the voltage of the virtual supply rail 6 (i.e. a glitch). In order to make the power-on controller 14 resistant to such glitches when the strong header transistors 10 are switched on, one possibility is to use a Schmitt trigger within the power-on controller 14 to detect the trigger voltage Vtrig. The hysteresis characteristic of such a Schmitt trigger will mean that once the strong header transistors 10 have been switched on when the trigger voltage Vtrig is reached, then they will not be switched off by the Schmitt trigger unless the voltage on the virtual supply rail 6 falls significantly below the trigger voltage Vtrig. This gives the power-on controller 14 a resistant to temporary dips in the virtual supply rail voltage.